At our local (small) Tesco there's 3 self service 'tills' as well as 2 normal tills and the queue for each is very distinctly separate/marked out but with the way they're laid out one of the till operators can see who's queuing for the self serve tills, and the queue for both is easily visible whichever one you're in.
A few times when I've been in there I've queued for the self serve tills because I want to be able to pack as I scan (not talking being ridiculously slow here, just not throwing the stuff through as fast as possible and not taking stuff out of the basket in a weird order so it's harder to pack), and also want to check the price of reduced/offer stuff is right. Did this yesterday and the staff member on the till finished with the customer she was serving and as there was no queue for her till called "next please" at the self serve queue. I was in front and gave her a polite smile and pointed to the self service machines to show I was happy waiting for them. She then shouted it twice more, sounding more pissy each time, with a real scowl, until the guy behind me went up to her (I'd said he could go ahead of me after the first time but didn't want to make him in case he was happy waiting too).
This has happened a few times before but I've usually gone up to the till or it's been covered by someone else rushing to the manned till. AIBU that it's very helpful to ask once if people want serving but after that it's incredibly rude to basically insist that people should go to you if they're not taking advantage of your offer? Really dislike having to hope the manned till(s) have a large queue if I want to be able to use the machine in peace!